---QUOTATION--- is there a mod that makes the game harder or more difficult i.e. more tougher bandits, zombies, monsters ect... ---END QUOTATION---
A few simple changes may be adequate, depending on what kind of difficulty you want. Making the player receive more damage, making bleeding hurt bad, making enemies stronger, or making enemies be more accurate is very easy to do.
I don't know how to add enemies or monsters to the game. I'm sure that someone here has made a mod with roaming hordes of belligerents; I will see if I can find a good one when I get my new computer built.